1. Field of the Invention
Embodiments of the present invention generally relate to distributed temperature sensing (DTS) and, more particularly, to correcting DTS measurements based on distributed loss measurements.
2. Description of the Related Art
Distributed Temperature Sensing (DTS) is a technique of monitoring temperature along the length of a wellbore utilizing an optical waveguide, such as an optical fiber, as a temperature sensor. In a typical DTS system, a laser or other light source at the surface of the well transmits a pulse of light into a fiber optic cable installed along the length of a well. Due to interactions with molecular vibrations within the glass of the fiber, a portion of the light is scattered back towards the surface (this phenomenon is referred to as Raman scattering).
FIG. 1 illustrates a conventional DTS system 100 for measuring the temperature in a well bore 110. A transmitter 102 irradiates a waveguide 120 with light signals (pump radiation) capable of causing Raman scattering. A coupler 104 includes suitable optical elements to guide pump radiation down the waveguide 120 and guide backscattered light signals to a receiver 106. The receiver 106 translates the backscattered light signals into electrical signals that are fed to a processor 108 capable of generating a distributed temperature profile therefrom.
FIG. 2 illustrates a waveform 202 across a spectrum of backscattered light signals generated by the pump radiation. As illustrated, the backscattered signals include signals in Brillouin Stokes and anti-Stokes bands, as well as Raman Stokes and anti-Stokes bands. The Raman Stokes and anti-Stokes signals are typically processed by the processor 108 at the surface to calculate a ratio of power between upper and lower frequency bands of detected signals.
There is a known temperature dependence of this ratio which allows for convenient temperature sensing based on the detected light signals scattered to the surface. The Raman anti-Stokes signal is sensitive to temperature changes, which result in changes in amplitude of the Raman anti-Stokes signal (as illustrated by the dashed line 204), while the Raman Stokes signal is insensitive to temperature. Because speed of light in the waveguide 120 is known, it is possible to determine positions along the fiber at which scattering occurred, based on the time of arrival of the backscattered light signals. Hence, a Raman DTS system is capable of measuring temperature as a continuous function of position over a length of the fiber, which may be correlated to a depth of the wellbore.
Unfortunately, DTS systems based on Raman scattering in an optical waveguide are susceptible to measurement errors due to differential (and/or varying) loss between the Stokes and anti-Stokes generated signals. In other words, due to this differential loss, the ratio calculated by the processor may not be accurate. The differential loss can be caused by any combination of several factors including uneven draw conditions resulting in variations along a fiber, environmental conditions such as hydrogen ingress, and installation conditions such as bending in the fiber that result in different optical responses at the Raman Stokes and anti-Stokes frequency bands.
This loss difference can be significant due to the large frequency separation of these two signals, which may be several THz (e.g., approximately 26 THz) in silica based optical fibers often used in downhole applications. While calibration for these differences may be carried out under some initial test conditions, these test conditions often bare little resemblance to the actual installation conditions. As an example, while a DTS system with several kilometers of fiber may be installed downhole, calibration is typically performed with the fiber on a spool in a temperature controlled oven.
Therefore, techniques and apparatus for correcting errors in DTS measurements caused by differential loss between Raman Stokes and anti-Stokes signals are needed.